1. Field
The present invention relates to fuel cells, in particular to fuel cells in operation of which an alcohol is supplied as fuel to the anode region of the cell. Such cells have applications in microfuel cells for electronic and portable electronic components, and also in larger fuel cells for the automotive industry.
2. Description of the Related Art
A fuel cell is an electrochemical energy conversion device that converts fuel and oxidant into reaction products, producing electricity and heat in the process. In one example of such a cell, methanol is used as fuel, and air or oxygen as oxidant, and the products of the reaction are carbon dioxide and water. The electrochemical reactions in this cell in operation may be summarised as follows:Anode CH3OH+H2O⇄CO2+6H++6e−Cathode 3/2O2+6H++6e−⇄3H2OOverall CH3OH+ 3/2O2⇄CO2+2H2O
The methanol fuel and oxidant are fed respectively into catalysing, diffusion-type electrodes separated by an electrolytic membrane which allows the passage of protons from the anode chamber to the cathode chamber to balance the cathode reaction. The electrons generated in the anode chamber flow in an external electrical circuit and are returned to the cathode having provided the power output from the cell. Such fuel cells are known as direct methanol fuel cells (DMFCs). Various types of membrane may be used, such as polymer electrolyte membranes (PEMs), comprising for example Nafion™. Fuel cells based on polymer electrolyte membranes (PEM fuel cells) are convenient for portable applications such as portable electronics and automotive technology due to their relatively low temperatures of operation. Further or alternative adaptations to the PEM barrier include the provision of a bimembrane as described in our co-pending application PCT/EP2006/060640.
WO-A-2006/012637 discloses a reactor and corresponding method for producing electrical energy using a fuel cell by selectively oxidising CO at room temperature using polyoxometallate compounds.
US-A-2005/0112055 discloses a catalyst comprising di-ruthenium-substituted polyoxometallate and a method of using the electrocatalyst to generate oxygen.
US-A-2004/0137297 discloses an ion exchange membrane said to be useful for the diaphragm of a direct methanol type fuel cell.
Methanol and other low molecular weight alcohols are convenient fuels for portable fuel cells because their energy density is relatively high, eg, for methanol, six moles of electrons being generated in the electrochemical half cell for every mole of fuel consumed. However, DMFCs typically suffer from crossover effects—methanol is transported across the membrane by diffusion and electro-osmosis. This causes a reduction in the performance of the fuel cell by the effect of methanol being oxidized at the cathode, typically comprising Pt or other noble metal catalyst. Here the methanol is oxidized at the potentials of oxygen reduction. The potential and current are reduced, causing a loss in power density; the open circuit potential is also reduced.
Conventionally, routes to reduce the methanol crossover effect have included:                i) Increasing the membrane thickness—typically 170 μm Nafion is used instead of the more common 50 μm membrane for hydrogen fuel cells. This increases the resistance of the membrane—whilst not completely eliminating the crossover impact.        ii) Using an alternative membrane to the Nafion-type sulphonated fluoropolymer. Usually these membranes require higher temperatures (>100° C.) to operate effectively, or conduct less well or swell.        iii) Using selective catalysts for the cathode. These are generally poorer catalysts for oxygen reduction than noble metal-containing catalysts such as Pt and Pt-containing catalysts.        
The phenomenon of methanol crossover and potential solutions have been reviewed recently:                International activities in DMFC R&D: status of technologies and potential applications, R Dillon, S Sriinivasan, A S Arico, V Antonucci, J Power Sources, 127, 112 (2004).        M P Hogarth, T R Ralph, Platinum Metal Reviews, 46, 146 (2002).        A Heinzel, V M Barragan, J Power Sources, 84, 70 (1999).        